The invention relates to an apparatus for optimizing the exhaust gas burn out in combustion plants with a solid bed combustion zone and an exhaust gas burn-out zone, comprising several controllable nozzles for introducing oxygen containing secondary air into the exhaust gas burn-out zone, wherein an oxygen measuring device and/or combustion chamber temperature measuring devices for determining the total amount of secondary and primary air in the exhaust gas are provided.
As a result of the highly heterogeneous composition of certain combustion material such as waste materials or bio-masses, the heat value of the combustion material varies greatly. In combustion systems with grate combustion therefore complicated and expensive combustion controls including infrared detectors (IR cameras, infrared cameras) are used. The combustion conditions in combustion chambers with grate combustion can be determined on the basis of the infra red radiation of the combustion material bed using an IR camera. The wavelength (3.9 μm) is in a range in which combustion gases have no emissivity. Using this information, the various primary gas flows are controlled which flow through the bed of solids. In this way, an essentially complete burn-out of the slag or bed ash can be achieved.
The exhaust gas leaving the combustion chamber (solid bed combustion zone) of such a non-uniform combustion includes areas with high concentrations of incompletely burned compounds such as CO, hydrocarbons or soot. The gas flow leaving the combustion chamber includes flow streaks with largely varying local and time dependent variations. These streaks of unburned exhaust gas components extend through the exhaust gas burnout zone up to the first radiation structure.
The oxygen concentrations in the exhaust gas burnout zone are very low and additionally, unevenly distributed. There is insufficient time and insufficient turbulence for a complete burn-out of the exhaust gases. A complete burn out of the exhaust gases can therefore be realized only with a controlled local introduction of secondary air into the exhaust gas burnout zone, wherein the secondary air must be mixed with the exhaust gas as well as possible.
Because of the inhomogeneity of the combustion material and the variations in the primary gas admitted to the solid bed combustion zone but also because of the different charges the spatial distribution and absolute concentrations of the exhaust gas species are distributed very heterogeneously and, additionally subject to strong fluctuations. Measurements, taken in the burnout zone, show air streaks with very high concentration of incompletely burned compounds. This results in an incomplete gas burnout with for example high CO peaks. In addition, particularly the incomplete burn out of soot particles, results in a high carbon concentration in the wall deposits an increased formation rate of PCDD/F (de-novo synthesis).
Technical apparatus for optimizing the exhaust gas burnout in combustion plants are designed particularly to reduce the emissions using controlled injection of oxygen-containing secondary gases which results in a reduction of emissions in the exhaust gas burnout zone formed in the exhaust gas discharge duct. As secondary gases for example more or less oxygen-containing air, recycled exhaust gas or also steam (with over-stoichiometric air) may be used.
In order to ensure a complete combustion secondary gas is injected into the exhaust gas burnout zone with a high impulse, and, to ensure good penetrations of the exhaust gas flow, in large excess quantities. The intense mixing of unburned exhaust gas components with the oxygen-containing secondary air at high temperatures is required for an effective exhaust gas burn out.
In [1] various concepts and apparatus for the injection of secondary air which are independent of local- and time-based condition changes, are described. The injection is performed in a first concept with nozzles, which are arranged exclusively around the burnout chamber wall. A turbulent mixing of the injected secondary air with the exhaust gas flow is attempted to be achieved by an optimal arrangement and orientation of the injection nozzles in the burnout chamber wall. It is consequently tried to obtain certain two- or three-dimensional flow patterns such as rolling flows or flow turbulences only by the arrangement and orientation of the nozzles. In a second concept, a cross-tube with additional nozzles is disposed in the narrowest flow cross-section that is in the transition from the combustion chamber to the radiation passage. A first variant of this concept uses a rotating tube, type Temelli, whereas a second variant is based on a flow-optimized stationary crosstube, type Kümmel.
A reliable mixing of secondary gas via injection nozzles which are arranged exclusively in the burnout chamber wall requires that certain flow patterns are maintained in order to obtain a homogenizing mixing process. Such concepts are therefore only conditionally suitable for instationary combustion processes as they occur for example in connection with the treatment of thermal waste materials. An inhomogeneous consistency of the waste material, which serves as fuel, amplifies this influence factor particularly strongly. This limitation is even more apparent with an increasing cross-section of the burn-out zone since the distances to be bridged by the injected secondary gas during the mixing process become larger.
It is therefore the object of the present invention to provide an apparatus and a method for optimizing the burn-out of exhaust gases such that even in instationary combustion processes a complete burn-out is achieved with a minimum of secondary gases.